Hollow objects promise a considerable potential for use in lightweight structures. In their technical applications, the weight of the material is utilized more efficiently for providing the desired function. Besides hollow objects in which the presence of a cavity is the necessary condition, e.g. in the design of piping, pressure vessels, heat exchangers or springs, the number of applications has been increasing recently in which a cavity is predominantly a feature that provides weight savings, including the aspect of the moment of inertia. Hollow rotating shafts may serve as an example. They have much lower mass than identical solid shafts. Yet, such hollow shafts can transmit a comparable torque at identical outside dimensions. In addition, their acceleration and deceleration require much less energy, owing to their lower moment of inertia. The better the mechanical properties of material, the thinner the wall can be—and the higher the efficiency of the mass of the structural element.
Hollow objects made of steels must be first converted to the required shape and then, in order to obtain excellent properties, heat treated to impart high strength and sufficient toughness. The shape of such a semi-finished product can be obtained by various methods, e.g. machining, forming or welding.
The weakness of the existing method of making hollow objects or semi-finished products is that it is problematic, technically demanding, complicated in materials terms and costly to achieve their desired shape and optimum properties. Moreover, conventional machining methods generate large quantities of waste in the form of chips. Conventional combinations of forming or other methods with subsequent treatment require multiple heating operations resulting in higher overall energy consumption in production. Some complex shapes, such as those without rotational-symmetry and with other than straight axis, are even impossible to manufacture by conventional methods. Prior art includes, for example, Czech Republic Patent No. 302917, which describes a method of manufacture of high-strength objects of multiphase martensitic steels. Making a hollow object comprises a heating process, a forming process and a cooling process. The input semi-finished product is heated to an austenite temperature of the material of which the semi-finished product is made, the semi-finished product is then converted to the final shape of the hollow object in a forming device and immediately after that this object is cooled to a temperature, at which incomplete transformation of austenite to martensite takes place. Immediately thereafter, retained austenite stabilization is effected in a heating device by way of diffusion-based carbon partitioning within the material from which the hollow object is made. Once the stabilization is finished, the hollow object is cooled in a cooling device to ambient temperature.